Multivalent Metal Ion Battery Having a Cathode of Recompressed Graphite Worms and Manufacturing Method

ABSTRACT

Provided is a multivalent metal-ion battery comprising an anode, a cathode, a porous separator electronically separating the anode and the cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of a multivalent metal, selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof, at the anode, wherein the anode contains the multivalent metal or its alloy as an anode active material and the cathode comprises a cathode layer of an exfoliated graphite or carbon material recompressed to form an active layer that is oriented in such a manner that the active layer has a graphite edge plane in direct contact with the electrolyte and facing or contacting the separator.

FIELD OF THE INVENTION

The present invention relates generally to the field of rechargeablemultivalent metal battery (e.g. zinc-, nickel-, magnesium-, orcalcium-ion battery) and, more particularly, to a high-capacity cathodeactive layer of exfoliated graphite or carbon worms and a method ofmanufacturing the multivalent metal-ion battery.

BACKGROUND OF THE INVENTION

Historically, today's most favorite rechargeable energy storagedevices—lithium-ion batteries—was actually evolved from rechargeable“lithium metal batteries” that use lithium (Li) metal as the anode and aLi intercalation compound (e.g. MoS₂) as the cathode. Li metal is anideal anode material due to its light weight (the lightest metal), highelectronegativity (−3.04 V vs. the standard hydrogen electrode), andhigh theoretical capacity (3,860 mAh/g). Based on these outstandingproperties, lithium metal batteries were proposed 40 years ago as anideal system for high energy-density applications.

Due to some safety concerns of pure lithium metal, graphite wasimplemented as an anode active material in place of the lithium metal toproduce the current lithium-ion batteries. The past two decades havewitnessed a continuous improvement in Li-ion batteries in terms ofenergy density, rate capability, and safety. However, the use ofgraphite-based anodes in Li-ion batteries has several significantdrawbacks: low specific capacity (theoretical capacity of 372 mAh/g asopposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g.low solid-state diffusion coefficients of Li in and out of graphite andinorganic oxide particles) requiring long recharge times (e.g. 7 hoursfor electric vehicle batteries), inability to deliver high pulse power,and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide,as opposed to cobalt oxide), thereby limiting the choice of availablecathode materials. Further, these commonly used cathode active materialshave a relatively low lithium diffusion coefficient (typically D˜10⁻¹⁶-10⁻¹¹ cm²/sec). These factors have contributed to one majorshortcoming of today's Li-ion batteries—a moderate energy density(typically 150-220 Wh/kg_(cell)) but extremely low power density(typically <0.5 kW/kg).

Supercapacitors are being considered for electric vehicle (EV),renewable energy storage, and modern grid applications. The relativelyhigh volumetric capacitance density of a supercapacitor (10 to 100 timesgreater than those of electrolytic capacitors) derives from using porouselectrodes to create a large surface area conducive to the formation ofdiffuse double layer charges. This electric double layer capacitance(EDLC) is created naturally at the solid-electrolyte interface whenvoltage is imposed. This implies that the specific capacitance of asupercapacitor is directly proportional to the specific surface area ofthe electrode material, e.g. activated carbon. This surface area must beaccessible by the electrolyte and the resulting interfacial zones mustbe sufficiently large to accommodate the EDLC charges.

This EDLC mechanism is based on surface ion adsorption. The requiredions are pre-existing in a liquid electrolyte and do not come from theopposite electrode. In other words, the required ions to be deposited onthe surface of a negative electrode (anode) active material (e.g.,activated carbon particles) do not come from the positive electrode(cathode) side, and the required ions to be deposited on the surface ofa cathode active material do not come from the anode side. When asupercapacitor is re-charged, local positive ions are deposited close toa surface of a negative electrode with their matting negative ionsstaying close side by side (typically via local molecular or ionicpolarization of charges). At the other electrode, negative ions aredeposited close to a surface of this positive electrode with the mattingpositive ions staying close side by side. Again, there is no exchange ofions between an anode active material and a cathode active material.

In some supercapacitors, the stored energy is further augmented bypseudo-capacitance effects due to some local electrochemical reactions(e.g., redox). In such a pseudo-capacitor, the ions involved in a redoxpair also pre-exist in the same electrode. Again, there is no exchangeof ions between the anode and the cathode.

Since the formation of EDLC does not involve a chemical reaction or anexchange of ions between the two opposite electrodes, the charge ordischarge process of an EDL supercapacitor can be very fast, typicallyin seconds, resulting in a very high power density (typically 3-10kW/Kg). Compared with batteries, supercapacitors offer a higher powerdensity, require no maintenance, offer a much higher cycle-life, requirea very simple charging circuit, and are generally much safer. Physical,rather than chemical, energy storage is the key reason for their safeoperation and extraordinarily high cycle-life.

Despite the positive attributes of supercapacitors, there are severaltechnological barriers to widespread implementation of supercapacitorsfor various industrial applications. For instance, supercapacitorspossess very low energy densities when compared to batteries (e.g., 5-8Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acidbattery and 50-100 Wh/kg for the NiMH battery). Modern lithium-ionbatteries possess a much higher energy density, typically in the rangeof 150-220 Wh/kg, based on the cell weight.

In addition to lithium-ion cells, there are several other differenttypes of batteries that are widely used in society: alkaline Zn/MnO₂,nickel metal hydride (Ni-MH), lead-acid (Pb acid), and nickel-cadmium(Ni—Cd) batteries. Since their invention in 1860, alkaline Zn/MnO₂batteries have become a highly popular primary (non-rechargeable)battery. It is now known that the Zn/MnO₂ pair can constitute arechargeable battery if an acidic salt electrolyte, instead of basic(alkaline) salt electrolyte, is utilized. However, the cycle life ofalkaline manganese dioxide rechargeable batteries has been limited totypically 20-30 cycles due to irreversibility associated with MnO₂ upondeep discharge and formation of electrochemically inactive phases.

Additionally, formation of a haeterolite (ZnO:Mn₂O₃) phase duringdischarge, when Zn penetrates into the lattice structure of MnO₂, hasmade battery cycling irreversible. The Zn anode also has limitations oncycle life due to the redistribution of Zn active material and formationof dendrites during recharge, causing internal short-circuits. Attemptsto solve some of these issues have been made by Oh, et al. [S. M. Oh,and S. H. Kim, “Aqueous Zinc Sulfate (II) Rechargeable Cell ContainingManganese (II) Salt and Carbon Powder,” U.S. Pat. No. 6,187,475, Feb.13, 2001] and by Kang, et al. [F. Kang, et al. “Rechargeable Zinc IonBattery”, U.S. Pat. No. 8,663,844, Mar. 4, 2014]. However, long-termcycling stability and power density issues remain to be resolved. Due tothese reasons, the commercialization of this battery has been limited.

Xu, et al. US Pub. No. 2016/0372795 (Dec. 22, 2016) and US Pub. No.2015/0255792 (Sep. 10, 2015) reported Ni-ion and Zn-ion cells,respectively, which both make use of graphene sheets or carbon nanotubes(CNTs) as the cathode active material. Although these two patentapplications claim an abnormally high specific capacity of 789-2500mAh/g based on the cathode active material weight, there are severalserious problems associated with these two cells:

-   (1) There is no plateau portion in the charge or discharge curves    (voltage vs. time or voltage vs. specific capacity), unlike typical    lithium-ion batteries. This lack of a voltage curve plateau means    the output voltage being non-constant (varying too much) and would    require a complicated voltage regulation algorithm to maintain the    cell output voltage at a constant level.-   (2) Actually, the discharge curve for the Ni-ion cell exhibits an    extremely sharp drop in voltage from 1.5 volts to below 0.6 volts as    soon as the discharge process begins and, during most of the    discharge process, the cell output is below 0.6 volts, which is not    very useful. As a point of reference, the alkaline cell (a primary    battery) provides an output voltage of 1.5 volts.-   (3) The discharge curves are characteristic of surface adsorption or    electroplating mechanisms at the cathode, as opposed to ion    intercalation. Further, it appears that the main event that occurs    at the cathode during the battery discharge is electroplating. The    high specific capacity values reported by Xu, et al. are simply a    reflection on the high amount of Ni or Zn metal electroplated on the    surfaces of graphene or CNTs. Since there is an excess amount of Ni    or Zn in the anode, the amount of electroplated metal increases as    the discharge time increases. Unfortunately, the electrochemical    potential difference between the anode and the cathode continues to    decrease since the difference in the metal amount between the anode    and the cathode continues to decrease (more Zn or Ni is dissolved    from the anode and gets electroplated on cathode surfaces). This is    likely why the cell output voltage continues to decrease. The cell    voltage output would be essentially zero when the amounts of metal    at the two electrodes are substantially equivalent or identical.    Another implication of this electroplating mechanism is the notion    that the total amount of the metal that can be deposited on the    massive surfaces at the cathode is dictated by the amount of the    metal implemented at the anode when the cell is made. The high    specific capacity (as high as 2,500 mAh/g) of graphene sheets at the    cathode simply reflects the excessively high amount of Zn provided    in the anode. There is no other reason or mechanism for why graphene    or CNTs could “store” so much metal. The abnormally high specific    capacity values as reported by Xu, et al. were artificially obtained    based on the high amounts of Ni or Zn electroplated on cathode    material surfaces, which unfortunately occurred at very low voltage    values and are of little utility value.

Clearly, an urgent need exists for new cathode materials that provideproper discharge voltage profiles (having a high average voltage and/ora high plateau voltage during discharge), high specific capacity at bothhigh and low charge/discharge rates (not just at a low rate), and longcycle-life for a multivalent metal secondary battery. Hopefully, theresulting battery can deliver some positive attributes of asupercapacitor (e.g. long cycle life and high power density) and somepositive features of a lithium-ion battery (e.g. moderate energydensity). These are the main objectives of the instant invention.

SUMMARY OF THE INVENTION

The invention provides a multivalent metal-ion battery comprising ananode, a cathode, and an electrolyte in ionic contact with the anode andthe cathode to support reversible deposition and dissolution of amultivalent metal (selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr,Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof) at theanode, wherein the anode contains the multivalent metal or its alloy asan anode active material and the cathode comprises a cathode activelayer of exfoliated graphite or carbon material having inter-flake poresfrom 2 nm to 10 μm in pore size. The multivalent metal alloy preferablycontains at least 80% by weight of the multivalent element in the alloy(more preferably at least 90% by weight). There is no restriction on thetype of alloying elements that can be chosen.

The exfoliated carbon or graphite material in the cathode active layermay be selected from a thermally exfoliated product of meso-phase pitch,meso-phase carbon, meso carbon micro-beads (MCMB), coke particles,expanded graphite flakes, artificial graphite particles, naturalgraphite particles, highly oriented pyrolytic graphite, soft carbonparticles, hard carbon particles, multi-walled carbon nanotubes, carbonnano-fibers, carbon fibers, graphite nano-fibers, graphite fibers,carbonized polymer fibers, or a combination thereof. Thesecarbon/graphite materials can be subjected to an expansion treatment(e.g. intercalation, oxidation, and/or fluorination), followed bythermal exfoliation to obtain exfoliated graphite/carbon worms.

The exfoliated graphite/carbon worms are preferably recompressed to forma cathode active layer of recompressed exfoliated graphite or carbonmaterial that is oriented in such a manner that the layer has a graphiteedge plane in direct contact with the electrolyte (to readily admit ionsfrom the electrolyte and release ions into electrolyte) and facing ortouching the separator (so that the ions permeating through the porousseparator can readily enter the inter-flake spaces near the edge plane).

In certain embodiments, the layer of recompressed exfoliated graphite orcarbon material has a physical density from 0.5 to 1.8 g/cm³ and hasmeso-scaled pores having a pore size from 2 nm to 50 nm. In somepreferred embodiments, the layer of recompressed exfoliated graphite orcarbon material has a physical density from 1.1 to 1.8 g/cm³ and haspores having a pore size from 2 nm to 5 nm. In certain embodiments, theexfoliated graphite or carbon material has a specific surface area from10 to 1,500 m²/g. The specific surface area of the recompressed worms istypically from 10 to 1,000 m²/g, but more typically and preferably from20 to 300 m²/g.

We have observed that a select multivalent metal (e.g. Ni, Zn, Be, Mg,Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Ga, In, or Cr), whencoupled with a presently invented layer of exfoliated graphite or carbonmaterial (preferably recompressed), can exhibit a discharge curveplateau at approximately 1.0 volt or higher (up to 3.7 volts). Thisplateau regime of a discharge voltage vs. time (or capacity) curveenables the battery cell to provide a useful constant voltage output. Avoltage output significantly lower than 1 volt is generally consideredundesirable. The specific capacity corresponding to this plateau regimeis typically from approximately 100 mAh/g to above 600 mAh/g.

This multivalent metal-ion battery can further comprise an anode currentcollector supporting the aluminum metal or aluminum metal alloy orfurther comprise a cathode current collector supporting the cathodeactive layer. The current collector can be a mat, paper, fabric, foil,or foam that is composed of conducting nano-filaments, such as graphenesheets, carbon nanotubes, carbon nano-fibers, carbon fibers, graphitenano-fibers, graphite fibers, carbonized polymer fibers, or acombination thereof, which form a 3D network of electron-conductingpathways. The high surface areas of such an anode current collector notonly facilitate fast and uniform dissolution and deposition of metalions, but also act to reduce the exchange current density and, thus, thetendency to form metal dendrites that otherwise could cause internalshorting.

The carbon or graphite material, such as meso-phase pitch, meso-phasecarbon, meso carbon micro-beads (MCMB), coke particles, expandedgraphite flakes, artificial graphite particles, natural graphiteparticles, highly oriented pyrolytic graphite, soft carbon particles,hard carbon particles, multi-walled carbon nanotubes, carbonnano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, andcarbonized polymer fibers, has an original inter-planar spacing d₀₀₂from 0.27 nm to 0.42 nm prior to a chemical or physical expansiontreatment and the inter-planar spacing d₀₀₂. is increased to from 0.43nm to 2.0 nm after the expansion treatment.

The expansion treatment includes an expansion treatment includes anoxidation, fluorination, bromination, chlorination, nitrogenation,intercalation, combined oxidation-intercalation, combinedfluorination-intercalation, combined bromination-intercalation, combinedchlorination-intercalation, or combined nitrogenation-intercalation ofthe graphite or carbon material. This expansion treatment is followed bythermal exfoliation and recompression. Due to the expansion treatments,the carbon or graphite material can contain a non-carbon elementselected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen,hydrogen, or boron.

Unconstrained thermal exfoliation typically results in exfoliatedgraphite/carbon worms that have inter-flake pores having an average sizefrom 20 nm to 50 μm (more typically from 100 nm to 10 μm). Theexfoliated graphite/carbon worms are then compressed to produce a layeror block of recompressed exfoliated carbon or graphite material that isoriented in such a manner that the layer has a graphite edge plane indirect contact with the electrolyte and facing or contacting theseparator. The recompressed exfoliated carbon or graphite materialtypically has a physical density from 0.5 to 1.8 g/cm³ and hasmeso-scaled pores having a pore size from 2 nm to 50 nm.

The electrolyte may be selected from an aqueous electrolyte, organicelectrolyte, polymer electrolyte, molten salt electrolyte, ionic liquidelectrolyte, or a combination thereof. In the invented multivalentmetal-ion battery, the electrolyte may contain NiSO₄, ZnSO₄, MgSO₄,CaSO₄, BaSO₄, FeSO₄, MnSO₄, CoSO₄, VSO₄, TaSO₄, CrSO₄, CdSO₄, GaSO₄,Zr(SO₄)₂, Nb₂(SO₄)₃, La₂(SO₄)₃, BeCl₂, BaCl₂, MgCl₂, AlCl₃, Be(ClO₄)₂,Ca(ClO₄)₂, Mg(ClO₄)₂, Mg(BF₄)₂, Ca(BF₄)₂, Be(BF₄)₂,tri(3,5-dimethylphenyl borane, tris(pentafluorophenyl)borane, AlkylGrignard reagents, magnesium dibutyldiphenyl, Mg(BPh2Bu2)2, magnesiumtributylphenyl Mg(BPhBu3)2), or a combination thereof.

In certain embodiments of the present invention, the electrolytecomprises at least a metal ion salt selected from a transition metalsulphate, transition metal phosphate, transition metal nitrate,transition metal acetate, transition metal carboxylate, transition metalchloride, transition metal bromide, transition metal perchlorate,transition metal hexafluorophosphate, transition metal borofluoride,transition metal hexafluoroarsenide, or a combination thereof.

In certain embodiments, the electrolyte comprises at least a metal ionsalt selected from a metal sulphate, phosphate, nitrate, acetate,carboxylate, chloride, bromide, or perchlorate of zinc, aluminum,titanium, magnesium, beryllium, calcium, manganese, cobalt, nickel,iron, vanadium, tantalum, gallium, chromium, cadmium, niobium,zirconium, lanthanum, or a combination thereof.

In the multivalent metal-ion battery, the electrolyte comprises anorganic solvent selected from ethylene carbonate (EC), dimethylcarbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC),methyl butyrate (MB), ethyl propionate, methyl propionate, propylenecarbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate(EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF),toluene, xylene, methyl acetate (MA), or a combination thereof.

In certain embodiments, the layer of exfoliated carbon or graphitematerial also operates as a cathode current collector to collectelectrons during a discharge of the battery and wherein the batterycontains no separate or additional cathode current collector.

The cathode active layer of exfoliated graphite/carbon may furthercomprise a binder material (preferably an electrically conductive bindermaterial), which bonds exfoliated graphite flakes together to form acathode electrode layer. The electrically conductive binder material maybe selected from coal tar pitch, petroleum pitch, meso-phase pitch, aconducting polymer, a polymeric carbon, or a derivative thereof.Non-conducting materials (e.g. PVDF, PTFE, SBR, etc.) may also be used.

Typically, the invented secondary battery has an average dischargevoltage typically no less than 1 volt (more typically andpreferably >1.5 volts) and a cathode specific capacity greater than 200mAh/g (preferably and more typically >300 mAh/g, more preferably >400mAh/g, and most preferably >500 mAh/g) based on a total cathode activelayer weight. Some cells deliver a specific capacity >600 mAh/g.

Preferably, the secondary battery has an average discharge voltage noless than 2.0 volts (preferably >2.5 volts, more preferably >3.0 volts,and most preferably >3.5 volts) and a cathode specific capacity greaterthan 100 mAh/g based on a total cathode active layer weight (preferablyand more typically >200 mAh/g, more preferably >300 mAh/g, and mostpreferably >500 mAh/g).

The present invention also provides a method of manufacturing amultivalent metal-ion battery. The method comprises: (a) providing ananode containing a multivalent metal (selected from Ni, Zn, Be, Mg, Ca,Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combinationthereof) or its alloy; (b) providing a cathode comprising a layer ofrecompressed exfoliated carbon or graphite material (recompressedgraphite/carbon worms or recompressed expanded graphite flakes, notgraphene sheets); and (c) providing an electrolyte capable of supportingreversible deposition and dissolution of the multivalent metal at theanode and reversible adsorption/desorption and/orintercalation/de-intercalation of ions at the cathode; wherein the layerof recompressed exfoliated carbon or graphite material is oriented insuch a manner that the layer has a graphite edge plane in direct contactwith the electrolyte and facing or contacting a plane of the separator.Typically, this graphite edge plane is substantially parallel to theporous separator and, thus, can readily admit and accommodate ions thatmigrate through the separator. Preferably, the electrolyte contains anaqueous electrolyte, an organic electrolyte, a molten salt electrolyte,or an ionic liquid.

The method can further include providing a porous network ofelectrically conductive nano-filaments to support the multivalent metalor its alloy at the anode.

In the method, the step of providing a cathode preferably containssubjecting a carbon or graphite material to an expansion treatmentselected from an oxidation, fluorination, bromination, chlorination,nitrogenation, intercalation, combined oxidation-intercalation, combinedfluorination-intercalation, combined bromination-intercalation, combinedchlorination-intercalation, or combined nitrogenation-intercalation,followed by thermal exfoliation at a temperature from 100° C. to 2,500°C.

In certain preferred embodiments, the procedure of providing the cathodeincludes compressing exfoliated graphite or carbon using a wetcompression or dry compression to align constituent graphite flakes ofthe exfoliated graphite or carbon. The procedure can produce a layer orblock of graphitic structure having a flake edge plane being parallel tothe separator, enabling direct entry of ions from separator pores intointer-flake spaces with minimal resistance. Preferably, the procedure ofproviding the cathode includes compressing exfoliated graphite or carbonusing a wet compression to align constituent graphite flakes of theexfoliated graphite or carbon, wherein wet compression includescompressing or pressing a suspension of exfoliated graphite or carbondispersed in a liquid electrolyte intended for use in a multivalentmetal-ion cell. Any of the aforementioned electrolytes can be utilizedin this suspension. The electrolyte later becomes part of theelectrolyte of the intended multivalent metal-ion battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic drawing illustrating the processes for producingintercalated and/or oxidized graphite, subsequently exfoliated graphiteworms, and conventional paper, mat, film, and membrane of simplyaggregated graphite or graphene flakes/platelets;

FIG. 1(B) An SEM image of exfoliated carbon (exfoliated carbon worms);

FIG. 1(C) An SEM image of graphite worms;

FIG. 1(D) Schematic drawing illustrating the approaches of producingthermally exfoliated graphite structures.

FIG. 1(E) A continuous process of producing recompressed exfoliatedgraphite, including feeding dry exfoliated graphite worms into the gapbetween a pair of two counter-rotating rollers or the gaps betweenseveral pairs of rollers.

FIG. 1(F) A schematic drawing to illustrate an example of a compressingand consolidating operation (using a mold cavity cell 302 equipped witha piston or ram 308) for forming a layer of highly compacted andoriented graphite flakes.

FIG. 1(G) Schematic drawing to illustrate another example of acompressing and consolidating operation (using a mold cavity cellequipped with a piston or ram) for forming a layer of highly compactedand oriented graphite flakes.

FIG. 2(A) Schematic of a multivalent metal secondary battery, whereinthe anode layer is a thin multivalent metal coating or foil and thecathode active material layer contains a layer of thermally exfoliatedgraphite/carbon; and

FIG. 2(B) Schematic of a multivalent metal secondary battery cell,wherein the anode layer is a thin multivalent metal coating or foil andthe cathode active material layer is composed of thermally exfoliatedgraphite/carbon, an optional conductive additive (not shown) and anoptional binder (not shown).

FIG. 3(A) The discharge curves of three Zn foil anode-based Zn-ioncells; first one containing a cathode layer of original graphiteparticles, second one containing a cathode layer of thermally exfoliatedgraphite (with only light recompression; pore size from 20 nm to 3 μm),and third one containing a cathode layer of thermally exfoliatedgraphite (with heavy recompression; pore size range from approximately 2nm to 20 nm; having an edge plane being parallel to the separator and inion-contact with the separator).

FIG. 3(B) The discharge curves of two Ca-ion cells; one containing acathode layer of thermally exfoliated graphite (with heavyrecompression) having an edge plane being parallel to the separator andin ionic contact with the separator); the other having a graphite flakesurface plane being parallel to the separator and in contact with theseparator. The cell containing a cathode of original artificial graphiteparticles exhibits a very short discharge curve (<30 mAh/g); not shownin the chart.

FIG. 4 The discharge curves of two Ni mesh anode-based cells; onecontaining a cathode layer of original MCMB particles and the other acathode layer of thermally exfoliated MCMB worms.

FIG. 5 The specific capacity of a Zn-MCMB cell (containing a cathodelayer of recompressed exfoliated MCMB) and a V-CNF (containing a cathodeof vapor grown carbon nanofibers exfoliated and recompressed), bothplotted as a function of the number of charge/discharge cycles.

FIG. 6 The specific capacity of a Mg-ion cell containing a cathode layerof heavily recompressed exfoliated MCMBs and favorably oriented; and thespecific capacity of a Mg-ion cell containing a cathode of lightlyrecompressed exfoliated MCMBs, both plotted as a function of the numberof charge/discharge cycles. The electrolyte used was 1 M of MgCl₂:AlCl₃(2:1) in monoglyme.

FIG. 7 Ragone plots of Mg-ion cells (electrolyte=1 M oftris(pentafluorophenyl)borane in tetrahydrofuran, 25° C.) and Ca-ioncells (electrolyte=0.45 M Ca(BF₄)₂ in EC:PC at 80° C.); the anode beingMg or Ca metal supported or unsupported by CNTs and graphene foam,respectively and the cathode being recompressed exfoliated artificialgraphite.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As schematically illustrated in the upper portion of FIG. 1(A), bulknatural graphite is a 3-D graphitic material with each graphite particlebeing composed of multiple grains (a grain being a graphite singlecrystal or crystallite) with grain boundaries (amorphous or defectzones) demarcating neighboring graphite single crystals. Each grain iscomposed of multiple graphene planes that are oriented parallel to oneanother. A graphene plane or hexagonal carbon atom plane in a graphitecrystallite is composed of carbon atoms occupying a two-dimensional,hexagonal lattice. In a given grain or single crystal, the grapheneplanes are stacked and bonded via van der Waal forces in thecrystallographic c-direction (perpendicular to the graphene plane orbasal plane). The inter-graphene plane spacing in a natural graphitematerial is approximately 0.3354 nm.

Artificial graphite materials also contain constituent graphene planes,but they have an inter-graphene planar spacing, d₀₀₂, typically from0.32 nm to 0.36 nm (more typically from 0.3339 to 0.3465 nm), asmeasured by X-ray diffraction. Many carbon or quasi-graphite materialsalso contain graphite crystals (also referred to as graphitecrystallites, domains, or crystal grains) that are each composed ofstacked graphene planes. These include meso-carbon mocro-beads (MCMBs),meso-phase carbon, soft carbon, hard carbon, coke (e.g. needle coke),carbon or graphite fibers (including vapor-grown carbon nano-fibers orgraphite nano-fibers), and multi-walled carbon nanotubes (MW-CNT). Thespacing between two graphene rings or walls in a MW-CNT is approximately0.27 to 0.42 nm. The most common spacing values in MW-CNTs are in therange of 0.32-0.35 nm, which do not strongly depend on the synthesismethod.

It may be noted that the “soft carbon” refers to a carbon materialcontaining graphite domains wherein the orientation of the hexagonalcarbon planes (or graphene planes) in one domain and the orientation inneighboring graphite domains are not too mis-matched from each other sothat these domains can be readily merged together when heated to atemperature above 2,000° C. (more typically above 2,500° C.). Such aheat treatment is commonly referred to as graphitization. Thus, the softcarbon can be defined as a carbonaceous material that can begraphitized. In contrast, a “hard carbon” can be defined as acarbonaceous material that contain highly mis-oriented graphite domainsthat cannot be thermally merged together to obtain larger domains; i.e.the hard carbon cannot be graphitized. Both hard carbon and soft carboncontain graphite domains that can be intercalated and thermallyexfoliated. The exfoliated carbon then can be recompressed to produce acathode layer having constituent graphite flakes being aligned.

The spacing between constituent graphene planes of a graphitecrystallite in a natural graphite, artificial graphite, and othergraphitic carbon materials in the above list can be expanded (i.e. thed₀₀₂ spacing being increased from the original range of 0.27-0.42 nm tothe range of 0.42-2.0 nm) using several expansion treatment approaches,including oxidation, fluorination, chlorination, bromination,iodization, nitrogenation, intercalation, combinedoxidation-intercalation, combined fluorination-intercalation, combinedchlorination-intercalation, combined bromination-intercalation, combinediodization-intercalation, or combined nitrogenation-intercalation of thegraphite or carbon material.

More specifically, due to the van der Waals forces holding the parallelgraphene planes together being relatively weak, graphite can be treatedso that the spacing between the graphene planes can be increased toprovide a marked expansion in the c-axis direction. This results in agraphite material having an expanded spacing, but the laminar characterof the hexagonal carbon layers is substantially retained. Theinter-planar spacing (also referred to as inter-graphene spacing) ofgraphite crystallites can be increased (expanded) via severalapproaches, including oxidation, fluorination, and/or intercalation ofgraphite. This is schematically illustrated in FIG. 1(D). The presenceof an intercalant, oxygen-containing group, or fluorine-containing groupserves to increase the spacing between two graphene planes in a graphitecrystallite and weaken the van der Waals forces between graphene planes,enabling easier thermal exfoliation.

The inter-planar spaces between certain graphene planes may besignificantly increased (actually, exfoliated) if the graphite/carbonmaterial having expanded d spacing is exposed to a thermal shock (e.g.by rapidly placing this carbon material in a furnace pre-set at atemperature of typically 800-2,500° C.) without constraint (i.e. beingallowed to freely increase volume). Under these conditions, thethermally exfoliated graphite/carbon material appears like worms,wherein each graphite worm is composed of many graphite flakes remaininginterconnected (please see FIG. 1(C)). However, these graphite flakeshave inter-flake pores typically in the pore size range of 20 nm to 10μm.

In one process, graphite materials having an expanded inter-planarspacing are obtained by intercalating natural graphite particles with astrong acid and/or an oxidizing agent to obtain a graphite intercalationcompound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(A). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes serves to increase the inter-graphenespacing, d₀₀₂, as determined by X-ray diffraction, thereby significantlyreducing the van der Waals forces that otherwise hold graphene planestogether along the c-axis direction. The GIC or GO is most oftenproduced by immersing natural graphite powder (100 in FIG. 1(A)) in amixture of sulfuric acid, nitric acid (an oxidizing agent), and anotheroxidizing agent (e.g. potassium permanganate or sodium perchlorate). Theresulting GIC (102) is actually some type of graphite oxide (GO)particles if an oxidizing agent is present during the intercalationprocedure. This GIC or GO is then repeatedly washed and rinsed in waterto remove excess acids, resulting in a graphite oxide suspension ordispersion, which contains discrete and visually discernible graphiteoxide particles dispersed in water.

Water may be removed from the suspension to obtain “expandablegraphite,” which is essentially a mass of dried GIC or dried graphiteoxide particles. The inter-graphene spacing, d₀₀₂, in the dried GIC orgraphite oxide particles is typically in the range of 0.42-2.0 nm, moretypically in the range of 0.5-1.2 nm. It may be noted than the“expandable graphite” is not “expanded graphite” (to be furtherexplained later). Graphite oxide can have an oxygen content of 2%-50% byweight, more typically 20%-40% by weight.

Upon exposure of expandable graphite to a temperature in the range oftypically 800-2,500° C. (more typically 900-1,050° C.) for approximately30 seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “exfoliated graphite” or “graphite worms”(104). Graphite worms are each a collection of exfoliated, but largelyun-separated graphite flakes that remain interconnected (FIGS. 1(B) and1(C)). In exfoliated graphite, individual graphite flakes (eachcontaining 1 to several hundred of graphene planes stacked together) arehighly spaced from one another, having a spacing of typically 2.0 nm-10μm. However, they remain physically interconnected, forming an accordionor worm-like structure.

Exfoliated graphite worms can be mechanically compressed to obtain“recompressed exfoliated graphite” for the purpose of densifying themass of exfoliated graphite worms, reducing inter-flake pore sizes orspaces, and aligning the orientation of the constituent flakes. (In someengineering applications, the graphite worms are extremely heavilycompressed to form flexible graphite sheets or foils 106 that typicallyhave a thickness in the range of 0.1 mm-0.5 mm.) In the instantinvention, as illustrated in the lower right portion of FIG. 1(D),exfoliated graphite worms are compressed to the extent that theconstituent graphite flakes are more or less parallel to one another andthe edges of these flakes define an edge plane of the resulting block orlayer of re-compressed graphite worms. Primary surfaces of some of thegraphite flakes (top or bottom surfaces) can constitute a flake surfaceplane (as opposed to the edge plane).

Alternatively, in graphite industry, one may choose to use alow-intensity air mill or shearing machine to simply break up thegraphite worms for the purpose of producing the so-called “expandedgraphite” flakes (108) which contain mostly graphite flakes or plateletsthicker than 100 nm (hence, not a nano material by definition). It isclear that the “expanded graphite” is not “expandable graphite” and isnot “exfoliated graphite worm” either. Rather, the “expandable graphite”can be thermally exfoliated to obtain “graphite worms,” which, in turn,can be subjected to mechanical shearing to break up the otherwiseinterconnected graphite flakes to obtain “expanded graphite” flakes.Expanded graphite flakes typically have the same or similar inter-planarspacing (typically 0.335-0.36 nm) of their original graphite. Expandedgraphite is not graphene either. Expanded graphite flakes have athickness typically greater than 100 nm; in contrast, graphene sheetstypically have a thickness smaller than 100 nm, more typically less than10 nm, and most typically less than 3 nm (single layer graphene is 0.34nm thick). In the present invention, expanded graphite flakes may alsobe compressed to form a layer of recompressed graphite having thedesired orientation.

Further alternatively, the exfoliated graphite or graphite worms may besubjected to high-intensity mechanical shearing (e.g. using anultrasonicator, high-shear mixer, high-intensity air jet mill, orhigh-energy ball mill) to form separated single-layer and multi-layergraphene sheets (collectively called NGPs, 112), as disclosed in ourU.S. application Ser. No. 10/858,814. Single-layer graphene can be asthin as 0.34 nm, while multi-layer graphene can have a thickness up to100 nm, but more typically less than 3 nm (commonly referred to asfew-layer graphene). Multiple graphene sheets or platelets may be madeinto a sheet of NGP paper (114) using a paper-making process.

It may be noted that the “expandable graphite” or graphite with expandedinter-planar spacing may also be obtained by forming graphite fluoride(GF), instead of GO. Interaction of F₂ with graphite in a fluorine gasat high temperature leads to covalent graphite fluorides, from (CF)_(n)to (C₂F)_(n), while at low temperatures graphite intercalation compounds(GIC) C_(x)F (2≤x≤24) form. In (CF)_(n) carbon atoms are sp3-hybridizedand thus the fluorocarbon layers are corrugated consisting oftrans-linked cyclohexane chairs. In (C₂F)_(n) only half of the C atomsare fluorinated and every pair of the adjacent carbon sheets are linkedtogether by covalent C—C bonds. Systematic studies on the fluorinationreaction showed that the resulting F/C ratio is largely dependent on thefluorination temperature, the partial pressure of the fluorine in thefluorinating gas, and physical characteristics of the graphiteprecursor, including the degree of graphitization, particle size, andspecific surface area. In addition to fluorine (F₂), other fluorinatingagents (e.g. mixtures of F₂ with Br₂, Cl₂, or I₂) may be used, althoughmost of the available literature involves fluorination with F₂ gas,sometimes in presence of fluorides.

We have observed that lightly fluorinated graphite, C_(x)F (2≤x≤24),obtained from electrochemical fluorination, typically has aninter-graphene spacing (d₀₀₂) less than 0.37 nm, more typically <0.35nm. Only when x in C_(x)F is less than 2 (i.e. 0.5≤x<2) can one observea d₀₀₂ spacing greater than 0.5 nm (in fluorinated graphite produced bya gaseous phase fluorination or chemical fluorination procedure). When xin C_(x)F is less than 1.33 (i.e. 0.5≤x<1.33) one can observe a d₀₀₂spacing greater than 0.6 nm. This heavily fluorinated graphite isobtained by fluorination at a high temperature (>>200° C.) for asufficiently long time, preferably under a pressure >1 atm, and morepreferably >3 atm. For reasons remaining unclear, electrochemicalfluorination of graphite leads to a product having a d spacing less than0.4 nm even though the product C_(x)F has an x value from 1 to 2. It ispossible that F atoms electrochemically introduced into graphite tend toreside in defects, such as grain boundaries, instead of between grapheneplanes and, consequently, do not act to expand the inter-graphene planarspacing.

The nitrogenation of graphite can be conducted by exposing a graphiteoxide material to ammonia at high temperatures (200-400° C.).Nitrogenation may also be conducted at lower temperatures by ahydrothermal method; e.g. by sealing GO and ammonia in an autoclave andthen increased the temperature to 150-250° C.

Compression or re-compression of exfoliated graphite worms or expandedgraphite flakes into a layer or block of recompressed exfoliatedgraphite having a preferred graphite flake orientation can beaccomplished by using several procedures, which can be classified intotwo broad categories: dry pressing/rolling or wet pressing/rolling. Thedry process entails mechanically pressing graphite worms or expandedgraphite flakes in one direction (uniaxial compression) without thepresence of a liquid medium. Alternatively, as schematically illustratedin FIG. 1(E), the process includes feeding dry exfoliated graphite worms30 into the gap between two counter-rotating rollers (e.g. 32 a and 32b) to form a slightly compressed layer of “re-compressed exfoliatedgraphite,” which are then further compressed to form a thinner layer offurther re-compressed exfoliated graphite (containing aligned graphiteflakes) by directing the material into the gap between another tworollers (e.g. 34 a and 34 b). If necessary, another pair or multiplepairs of rollers (e.g. 36 a and 36 b) can be implemented to furtherreduce the layer thickness and further improve the degree of flakeorientation, resulting in a layer 38 of relatively well-alignedrecompressed exfoliated graphite.

A layer of oriented, recompressed exfoliated graphite structure (ormultiple layers of such a structure stacked and/or bonded together) maybe cut and slit to produce a desired number of pieces of the oriented,recompressed exfoliated graphite structure. Assuming that each piece isa cube or tetragon, each cube will then have 4 graphite flake edgeplanes and 2 flake surface planes as illustrated in the bottom rightportion of FIG. 1(D). When such a piece is implemented as a cathodelayer, the layer can be positioned and aligned in such a manner that oneof the flake edge planes is substantially parallel to the anode layer orthe porous separator layer. This flake edge plane typically is veryclose to or actually in direct contact with the separator layer. Such anorientation is found to be conducive to entry and exiting of ionsinto/from the electrode, leading to significantly improved high-ratecapability and high power density.

It may be noted that the same procedures can be used to produce a wetlayer of recompressed exfoliated graphite provided the starting materialcontains graphite worms dispersed in a liquid medium. This liquid mediummay be simply water or solvent, which must be removed upon completion ofthe roll-pressing procedure. The liquid medium may be or may contain aresin binder that helps to bond together exfoliated graphite worms orflakes, although a resin binder is not required or desired.Alternatively and desirably, some amount of the liquid electrolyte(intended to become part of the electrolyte of the final multivalentmetal-ion cell) may be mixed with the exfoliated graphite worms orexpanded graphite flakes prior to being compressed or roll-pressed.

The present invention also provides a wet process for producing anelectrolyte-impregnated recompressed graphite structure for use as amultivalent metal-ion battery cathode layer. In a preferred embodiment,the wet process (method) comprises: (a) preparing a dispersion or slurryhaving exfoliated graphite worms or expanded graphite flakes dispersedin a liquid or gel electrolyte; and (b) subjecting the suspension to aforced assembly procedure, forcing the exfoliated graphite worms orexpanded graphite flakes to assemble into the electrolyte-impregnatedre-compressed graphite structure, wherein electrolyte resides in theinter-flake spaces in recompressed exfoliated graphite. The graphiteflakes of the exfoliated graphite or the expanded graphite flakes aresubstantially aligned along a desired direction. The recompressedgraphite structure has a physical density from 0.5 to 1.7 g/cm³ (moretypically 0.7-1.3 g/cm³) and a specific surface area from 20 to 1,500m²/g, when measured in a dried state of the recompressed graphitestructure with the electrolyte removed.

In some desired embodiments, the forced assembly procedure includesintroducing an exfoliated graphite suspension, having an initial volumeV₁, in a mold cavity cell and driving a piston into the mold cavity cellto reduce the suspension volume to a smaller value V₂, allowing excesselectrolyte to flow out of the cavity cell (e.g. through holes of themold cavity cell or of the piston) and aligning the multiple graphiteflakes along a direction at an angle from approximately 45° to 90°relative to the movement direction of the piston. It may be noted thatthe electrolyte used in this suspension becomes portion of theelectrolyte for the intended multivalent metal-ion cell.

FIG. 1(F) provides a schematic drawing to illustrate an example of acompressing and consolidating operation (using a mold cavity cell 302equipped with a piston or ram 308) for forming a layer of highlycompacted and oriented graphite flakes 314. Contained in the chamber(mold cavity cell 302) is a suspension (or slurry) that is composed ofgraphite flakes 304 randomly dispersed in a liquid or gel electrolyte306. As the piston 308 is driven downward, the volume of the suspensionis decreased by forcing excess liquid electrolyte to flow through minutechannels 312 on a mold wall or through small channels 310 of the piston.These small channels can be present in any or all walls of the moldcavity and the channel sizes can be designed to permit permeation of theelectrolyte species, but not the solid graphite flakes. The excesselectrolyte is shown as 316 a and 316 b on the right diagram of FIG.1(E). As a result of this compressing and consolidating operation,graphite flakes 314 are aligned parallel to the bottom plane orperpendicular to the layer thickness direction.

Shown in FIG. 1(G) is a schematic drawing to illustrate another exampleof a compressing and consolidating operation (using a mold cavity cellequipped with a piston or ram) for forming a layer of highly compactedand oriented graphite flakes 320. The piston is driven downward alongthe Y-direction. The graphite flakes are aligned on the X-Z plane andperpendicular to X-Y plane (along the Z- or thickness direction). Thislayer of oriented graphite flakes can be attached to a current collector(e.g. graphene mat) that is basically represented by the X-Y plane. Inthe resulting electrode, graphite flakes are aligned perpendicular tothe current collector. Such an orientation is conducive to a faster ionintercalation into and out of the spaces between graphite flakes and,hence, leads to a higher power density as compared to the correspondingelectrode featuring graphite flakes being aligned parallel to thecurrent collector plane.

The configuration of a multivalent metal secondary battery is nowdiscussed as follows:

A multivalent metal-ion battery includes a positive electrode (cathode),a negative electrode (anode), and an electrolyte typically including ametal salt and a solvent. The anode can be a thin foil or film of amultivalent metal or its alloy with another element(s); e.g. 0-10% byweight of Sn in Zn. The multivalent metal may be selected from Ni, Zn,Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or acombination thereof. The anode can be composed of particles, fibers,wires, tubes, or discs of the multivalent metal or metal alloy that arepacked and bonded together by a binder (preferably a conductive binder)to form an anode layer.

We have observed that a select multivalent metal (e.g. Ni, Zn, Be, Mg,Ca, Ba, La, Ti, Ta, Zr, Mn, V, Co, Fe, Cd, Ga, or Cr), when coupled witha presently invented graphite or carbon material having expandedinter-graphene planar spaces, can exhibit a discharge curve plateau oraverage output voltage at approximately 1.0 volt or higher (up to 3.5volts). This plateau regime of a discharge voltage vs. time (orcapacity) curve enables the battery cell to provide a useful constantvoltage output. A voltage output lower than 1 volt is generallyconsidered as undesirable. The specific capacity corresponding to thisplateau regime is typically from approximately 100 mAh/g (e.g. for Zr orTa) to above 600 mAh/g (e.g. for Zn or Mg).

A desirable anode layer structure is composed of a network ofelectron-conducting pathways (e.g. mat of graphene sheets, carbonnano-fibers, or carbon-nanotubes) and a thin layer of the multivalentmetal or alloy coating deposited on surfaces of this conductive networkstructure. Such an integrated nano-structure may be composed ofelectrically conductive nanometer-scaled filaments that areinterconnected to form a porous network of electron-conducting pathscomprising interconnected pores, wherein the filaments have a transversedimension less than 500 nm. Such filaments may comprise an electricallyconductive material selected from the group consisting of electro-spunnano fibers, vapor-grown carbon or graphite nano fibers, carbon orgraphite whiskers, carbon nano-tubes, nano-scaled graphene platelets,metal nano wires, and combinations thereof. Such a nano-structured,porous supporting material for the multivalent metal can significantlyimprove the metal deposition-dissolution kinetics at the anode, enablinghigh-rate capability of the resulting multivalent metal secondary cell.

Illustrated in FIG. 2(A) is a schematic of a multivalent metal secondarybattery, wherein the anode layer is a thin multivalent metal coating orfoil and the cathode active material layer contains a layer of thermallyexfoliated graphite/carbon material that has been recompressed.Alternatively, FIG. 2(B) shows a schematic of a multivalent metalsecondary battery cell wherein the cathode active material layercontains a layer of thermally exfoliated graphite/carbon material thathas been recompressed, an optional conductive additive (not shown), anda resin binder (not shown) that helps to bond the worms together to forma cathode active layer of structural integrity.

The recompressed exfoliated graphite or carbon materials, whenimplemented as a cathode active material, enable the multivalentmetal-ion cell to exhibit a voltage plateau portion in a dischargevoltage-time or voltage-capacity curve obtained at a constant currentdensity. This plateau portion typically occurs at a relatively highvoltage value intrinsic to a given multivalent metal, and typicallylasts a long time, giving rise to a high specific capacity. Typically,this plateau portion is followed by a slopping curve portion,corresponding to a supercapacitor-type behavior. The supercapacitor-typebehavior (EDLC or redox) is due to the high specific surface area of theexfoliated graphite/carbon worms used in the cathode layer. In general,the plateau portion is increased and slopping curve portion decreasedwhen the degree of re-compression of worms is increased.

The composition of the electrolyte, which functions as anion-transporting medium for charge-discharge reaction, has a greateffect on battery performance. To put multivalent metal secondarybatteries to practical use, it is necessary to allow metaldeposition-dissolution reaction to proceed smoothly and sufficientlyeven at relatively low temperature (e.g., room temperature).

In the invented multivalent metal-ion battery, the electrolyte typicallycontains a metal salt dissolved in a liquid solvent. The solvent can bewater, organic liquid, ionic liquid, organic-ionic liquid mixture, etc.In certain desired embodiments, the metal salt may be selected fromNiSO₄, ZnSO₄, MgSO₄, CaSO₄, BaSO₄, FeSO₄, MnSO₄, CoSO₄, VSO₄, TaSO₄,CrSO₄, CdSO₄, GaSO₄, Zr(SO₄)₂, Nb₂(SO₄)₃, La₂(SO₄)₃, MgCl₂, AlCl₃,Mg(ClO₄)₂, Mg(BF₄)₂, Grignard reagents, magnesium dibutyldiphenyl,Mg(BPh2Bu2)2, magnesium tributylphenyl Mg(BPhBu3)2), or a combinationthereof.

The electrolyte may in general comprise at least a metal ion saltselected from a transition metal sulphate, transition metal phosphate,transition metal nitrate, transition metal acetate, transition metalcarboxylate, transition metal chloride, transition metal bromide,transition metal nitride, transition metal perchlorate, transition metalhexafluorophosphate, transition metal borofluoride, transition metalhexafluoroarsenide, or a combination thereof.

In certain embodiments, the electrolyte comprises at least a metal ionsalt selected from a metal sulphate, phosphate, nitrate, acetate,carboxylate, chloride, bromide, nitride, or perchlorate of zinc,aluminum, titanium, magnesium, calcium, manganese, cobalt, nickel, iron,vanadium, tantalum, gallium, chromium, cadmium, niobium, zirconium,lanthanum, or a combination thereof.

In the multivalent metal-ion battery, the electrolyte comprises anorganic solvent selected from ethylene carbonate (EC), dimethylcarbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC),methyl butyrate (MB), ethyl propionate, methyl propionate, propylenecarbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate(EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF),toluene, xylene, methyl acetate (MA), or a combination thereof.

This invention is directed at the cathode active layer (positiveelectrode layer) containing a high-capacity cathode material for themultivalent metal secondary battery. The invention also provides such abattery based on an aqueous electrolyte, a non-aqueous electrolyte, amolten salt electrolyte, a polymer gel electrolyte (e.g. containing ametal salt, a liquid, and a polymer dissolved in the liquid), an ionicliquid electrolyte, or a combination thereof. The shape of a multivalentmetal secondary battery can be cylindrical, square, button-like, etc.The present invention is not limited to any battery shape orconfiguration.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention.

EXAMPLE 1 Oxidation of Graphite and Thermal Exfoliation of OxidizedGraphite

Natural flake graphite, nominally sized at 45 μm, provided by AsburyCarbons (405 Old Main St., Asbury, N.J. 08802, USA) was milled to reducethe size to approximately 14 μm (Sample 1a). The chemicals used in thepresent study, including fuming nitric acid (>90% concentration),sulfuric acid (95-98%), potassium chlorate (98%), and hydrochloric acid(37%), were purchased from Sigma-Aldrich and used as received. Graphiteoxide (GO) samples were prepared according to the following procedure:

Sample 1A: A reaction flask containing a magnetic stir bar was chargedwith sulfuric acid (176 mL) and nitric acid (90 mL) and cooled byimmersion in an ice bath. The acid mixture was stirred and allowed tocool for 15 min, and graphite (10 g) was added under vigorous stirringto avoid agglomeration. After the graphite powder was well dispersed,potassium chlorate (110 g) was added slowly over 15 min to avoid suddenincreases in temperature. The reaction flask was loosely capped to allowevolution of gas from the reaction mixture, which was stirred for 24hours at room temperature. On completion of the reaction, the mixturewas poured into 8 L of deionized water and filtered. The GO wasre-dispersed and washed in a 5% solution of HCl to remove sulphate ions.The filtrate was tested intermittently with barium chloride to determineif sulphate ions are present. The HCl washing step was repeated untilthis test was negative. The GO was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The GO slurry wasspray-dried and stored in a vacuum oven at 60° C. until it was used.

Sample 1B: The same procedure as in Sample 1A was followed, but thereaction time was 48 hours.

Sample 1C: The same procedure as in Sample 1A was followed, but thereaction time was 96 hours.

X-ray diffraction studies showed that after a treatment of 24 hours, asignificant proportion of graphite has been transformed into graphiteoxide. The peak at 2θ=26.3 degrees, corresponding to an inter-planarspacing of 0.335 nm (3.35 Å) for pristine natural graphite wassignificantly reduced in intensity after a deep oxidation treatment for24 hours and a peak typically near 2θ=9-14 degrees (depending upondegree of oxidation) appeared. In the present study, the curves fortreatment times of 48 and 96 hours are essentially identical, showingthat essentially all of the graphite crystals have been converted intographite oxide with an inter-planar spacing of 6.5-7.5 Å (the 26.3degree peak has totally disappeared and a peak of approximately at2θ=11.75-13.7 degrees appeared).

Samples 1A, 1B, and 1C were then subjected to unconstrained thermalexfoliation (1,050° C. for 2 minutes) to obtain thermally exfoliatedgraphite worms. The graphite worms were compressed into layers oforiented, recompressed exfoliated graphite having physical densityranging from approximately 0.5 to 1.75 g/cm³, using both dry compressionand wet compression procedures.

EXAMPLE 2 Oxidation and Intercalation of Various Graphitic Carbon andGraphite Materials

Samples 2A, 2B, 2C, and 2D were prepared according to the same procedureused for Sample 1B, but the starting graphite materials were pieces ofhighly oriented pyrolytic graphite (HOPG), graphite fiber, graphiticcarbon nano-fiber, and spheroidal graphite, respectively. Their finalinter-planar spacing values are 6.6 Å, 7.3 Å, 7.3 Å, and 6.6 Å,respectively. Their un-treated counterparts are referred to as Sample2a, 2b, 2c, and 2d, respectively. The treated samples were subsequentlythermally exfoliated and recompressed to obtain samples of variouscontrolled densities, specific surface areas, and degrees oforientation.

EXAMPLE 3 Preparation of Graphite Oxide Using a Modified Hummers' Methodand Subsequent Thermal Exfoliation

Graphite oxide (Sample 3A) was prepared by oxidation of natural graphiteflakes (original size of 200 mesh, milled to approximately 15 μm,referred to as Sample 3a) with sulfuric acid, sodium nitrate, andpotassium permanganate according to the method of Hummers [U.S. Pat. No.2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite,we used a mixture of 22 ml of concentrated sulfuric acid, 2.8 grams ofpotassium permanganate, and 0.5 grams of sodium nitrate. The graphiteflakes were immersed in the mixture solution and the reaction time wasapproximately one hour at 35° C. It is important to caution thatpotassium permanganate should be gradually added to sulfuric acid in awell-controlled manner to avoid overheat and other safety issues. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The sample was then washed repeatedly with deionized wateruntil the pH of the filtrate was approximately 5. The slurry wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theinterlayer spacing of the resulting laminar graphite oxide wasdetermined by the Debye-Scherrer X-ray technique to be approximately0.73 nm (7.3 Å). Some of the powder was subsequently exfoliated in afurnace, pre-set at 950-1,100° C., for 2 minutes to obtain thermallyexfoliated graphite worms. The graphite worms were recompressed usingboth the wet and dry press-rolling procedures to obtain oriented,recompressed exfoliated graphite worms.

EXAMPLE 4 Oxidation and Thermal Exfoliation of Meso-Carbon Micro-Beads(MCMBs)

Graphite oxide (Sample 4A) was prepared by oxidation of meso-carbonmicro-beads (MCMBs) according to the same procedure used in Example 3.MCMB microbeads (Sample 4a) were supplied by China Steel Chemical Co.This material has a density of about 2.24 g/cm³, an average particlesize of 16 μm, and an inter-planar distance of about 0.336 nm. Afterdeep oxidation treatment, the inter-planar spacing in the resultinggraphite oxide micro-beads is approximately 0.76 nm. The treated MCMBswere then thermally exfoliated at 900° C. for 2 minutes to obtainexfoliated carbon, which also showed a worm-like appearance (hereinreferred to as “exfoliated carbon”, “carbon worms,” or “exfoliatedcarbon worms”). The carbon worms were then roll-pressed to differentextents to obtain recompressed exfoliated carbon having differentdensities, specific surface areas, and degrees of orientation.

EXAMPLE 5 Bromination and Fluorination of Carbon Fibers

Graphitized carbon fiber (Sample 5a), having an inter-planar spacing of3.37 Å (0.337 nm) and a fiber diameter of 10 μm was first halogenatedwith a combination of bromine and iodine at temperatures ranging from75° C. to 115° C. to form a bromine-iodine intercalation compound ofgraphite as an intermediate product. The intermediate product was thenreacted with fluorine gas at temperatures ranging from 275° C. to 450°C. to form the CF_(y). The value of y in the CF_(y) samples wasapproximately 0.6-0.9. X-ray diffraction curves typically show theco-existence of two peaks corresponding to 0.59 nm and 0.88 nm,respectively. Sample 5A exhibits substantially 0.59 nm peak only andSample 5B exhibits substantially 0.88 nm peak only. Some of powders werethermally exfoliated and then re-compressed to obtain oriented,recompressed exfoliated graphite.

EXAMPLE 6 Fluorination and Thermal Exfoliation of Carbon Fibers

A CF_(0.68) sample obtained in EXAMPLE 5 was exposed at 250° C. and 1atmosphere to vapors of 1,4-dibromo-2-butene (BrH₂C—CH═.CH—CH₂Br) for 3hours. It was found that two-thirds of the fluorine was lost from thegraphite fluoride sample. It is speculated that 1,4-dibromo-2-buteneactively reacts with graphite fluoride, removing fluorine from thegraphite fluoride and forming bonds to carbon atoms in the graphitelattice. The resulting product (Sample 6A) is mixed halogenatedgraphite, likely a combination of graphite fluoride and graphitebromide. Some of powders were thermally exfoliated to obtain exfoliatedcarbon fibers, which were then roll-pressed to obtain oriented,recompressed exfoliated graphite worms.

EXAMPLE 7 Fluorination and Thermal Exfoliation of Graphite

Natural graphite flakes, a sieve size of 200 to 250 mesh, were heated invacuum (under less than 10⁻² mmHg) for about 2 hours to remove theresidual moisture contained in the graphite. Fluorine gas was introducedinto a reactor and the reaction was allowed to proceed at 375° C. for120 hours while maintaining the fluorine pressure at 200 mmHg. This wasbased on the procedure suggested by Watanabe, et al. disclosed in U.S.Pat. No. 4,139,474. The powder product obtained was black in color. Thefluorine content of the product was measured as follows: The product wasburnt according to the oxygen flask combustion method and the fluorinewas absorbed into water as hydrogen fluoride. The amount of fluorine wasdetermined by employing a fluorine ion electrode. From the result, weobtained a GF (Sample 7A) having an empirical formula (CF_(0.75))_(n).X-ray diffraction indicated a major (002) peak at 2θ=13.5 degrees,corresponding to an inter-planar spacing of 6.25 Å. Some of the graphitefluoride powder was thermally exfoliated to form graphite worms, whichwere then roll-pressed.

Sample 7B was obtained in a manner similar to that for Sample 7A, but ata reaction temperature of 640° C. for 5 hours. The chemical compositionwas determined to be (CF_(0.93))_(n). X-ray diffraction indicated amajor (002) peak at 2θ=9.5 degrees, corresponding to an inter-planarspacing of 9.2 Å. Some of the graphite fluoride powder was thermallyexfoliated to form graphite worms, which were then roll-pressed toproduce recompressed exfoliated graphite material.

EXAMPLE 8 Preparation and Testing of Various Multivalent Metal-Ion Cells

The exfoliated carbon/graphite materials prepared in Examples 1-7 wereseparately made into a cathode layer and incorporated into a multivalentmetal secondary battery. Two types of multivalent metal anode wereprepared. One was metal foil having a thickness from 20 μm to 300 μm.The other was metal thin coating deposited on surfaces of conductivenano-filaments (e.g. CNTs) or graphene sheets that form an integrated 3Dnetwork of electron-conducting pathways having pores and pore walls toaccept a multivalent metal or its alloy. Either the metal foil itself orthe integrated 3D nano-structure also serves as the anode currentcollector.

Cyclic voltammetry (CV) measurements were carried out using an Arbinelectrochemical workstation at a typical scanning rate of 0.5-50 mV/s.In addition, the electrochemical performances of various cells were alsoevaluated by galvanostatic charge/discharge cycling at a current densityfrom 50 mA/g to 10 A/g. For long-term cycling tests, multi-channelbattery testers manufactured by LAND were used.

FIG. 3(A) shows the discharge curves of three Zn foil anode-based Zn-ioncells: first one containing a cathode layer of original graphiteparticles, second one containing a cathode layer of thermally exfoliatedgraphite (with only light recompression; pore size from 20 nm to 3 μm),and third one containing a cathode layer of thermally exfoliatedgraphite (with heavy recompression; pore size range from approximately 2nm to 20 nm; having an edge plane being parallel to the separator and incontact with the separator). The electrolyte used was 1M of ZnSO₄ inwater. These data indicate that the original graphite has very littleion storage capability; the non-zero, but minimal capability beinglikely associated with surface adsorption or electroplating of Zn ongraphite particle surfaces (specific capacity <5 m²/g). In contrast, thelayer of heavily recompressed worms, when properly oriented, appear tobe capable of admitting and storing large amounts of Zn ions, possiblyalong with other ions dissociated from the electrolyte. The dischargecurve exhibits a long plateau regime at 1.20-1.35 volts and a specificcapacity as high as nearly 260 mAh/g. The resulting cell-level energydensity is approximately 120 Wh/kg, very close to the energy densitiesof lithium-ion batteries. However, Zinc is more abundant, safer, andsignificantly less expensive than lithium. The layer of lightlyrecompressed worms, having somewhat randomly oriented graphite flakes(with respect to the separator plane) only delivers a moderate chargestorage capability.

FIG. 3(B) shows discharge curves of two Ca-ion cells: one containing acathode layer of thermally exfoliated graphite (with heavyrecompression) having a flake edge plane being parallel to the separatorand in ionic contact with the separator); the other having a graphiteflake surface plane being parallel to the separator and in contact withthe separator. The cell containing a cathode of original artificialgraphite particles exhibits a very short discharge curve (<30 mAh/g);not shown in the chart. These data indicate that the presently inventedcathode layer of oriented recompressed graphite worms exhibits aninitial plateau regime corresponding to ion intercalation into thegraphitic structure and a slopping curve portion corresponding toadsorption of Ca ions or other electrolyte-derived ions on graphiteflake surfaces. The cell has a significantly higher specific capacity(250 mAh/g) as compared to the cathode featuring unfavorably orientedcompressed worms (70 mAh/g). The latter is only capable of storing Caions via the surface adsorption or electroplating mechanism.

Summarized in Table 1 below are the typical plateau voltage ranges ofthe discharge curves of a broad array of multivalent metal-ion cellsusing natural graphite, artificial graphite, or graphite fiber havingexpanded d-spacing as a cathode active material. The specific capacityis typically from 100 to 650 mAh/g. In contrast, for each type ofbattery cell, the corresponding original graphite or carbon materialdoes not enable any significant voltage plateau regime and does notprovide any significant ion storage capability (typically <50 mAh/g).

TABLE 1 Plateau voltage ranges of the discharge curves in multivalentmetal-ion cells. Anode Metal Voltage range Ba 3.40-3.55 V Ca 3.25-3.35 VLa 2.91-3.05 V Mg 2.85-3.01 V Be 2.40-2.51 V Ti 2.18-2.22 V Zr 1.98-2.07V Mn 1.78-1.85 V V 1.75-1.82 V Nb 1.67-1.73 V Zn 1.20-1.35 V Cr1.16-1.31 V Ta 1.17-1.25 V Ga 1.09-1.18 V Fe 0.96-1.13 V Cd 0.95-1.10 VCo 0.87-0.98 V Ni 0.85-0.95 V

Shown in FIG. 4 are the discharge curves of two Ni mesh anode-basedNi-ion cells, one containing a cathode layer of original MCMB particlesand the other a cathode layer of recompressed exfoliated MCMBs. Therecompressed MCMB worms enable the MCMB to admit (via intercalation andthen surface adsorption) and store a large amount of ions (up to 450mAh/g). In contrast, the original MCMB beads with unexpandedinter-planar spacing stores a very limited amount of Ni ions, mostly dueto electroplating at very low voltage levels. There is typically veryshort or no plateau regime in a charge or discharge curve for amultivalent metal-graphite/carbon cell having untreated, originalcarbon/graphite as the cathode active material.

The carbon or graphite material types, their respective inter-planarspacing values (prior to thermal exfoliation) and specific capacityvalues of the original graphite/carbon or exfoliated/recompressed wormswhen used as a cathode active material for Zn-ion, V-ion, and Mg-ioncells are summarized in Table 2 below:

TABLE 2 A list of carbon or graphite materials used as the cathodeactive material of an Al cell. Specific Specific Specific SampleInter-planar capacity, capacity, capacity, No. Material spacing, Å mAh/g(Zn) mAh/g (V) mAh/g (Mg) 1a Natural graphite 3.35 22 25 1A GO, 24 hrs5.5 223 211 1B GO, 48 hrs 7 301 287 1C GO, 96 hrs 7.6 346 269 2a HOPG3.35 21 27 2A HOPG oxide 6.6 287 204 2b Graphite fiber 3.4 16 32 2BOxidized GF 7.3 323 205 2c CNF 3.36 45 77 2C Oxidized CNF 7.3 332 277 3aNatural graphite 3.35 17 3A GO, Hummers 7.3 214 4a MCMB 3.36 22 4AOxidized MCMB 7.6 261 5a Graphite fiber 3.4 16 5A CF_(0.9) 8.8 343 5BCF_(0.6) 5.9 204 6A CBrF_(x) 8.4 325 7A CF_(0.75) 5.85 202 7B CF_(0.93)9.2 406

The following significant observations are made from Table 1 and relatedcharts (FIG. 5-FIG. 7):

(1) In every group of carbon or graphite material used in the cathode ofa multivalent metal-ion battery, the specific capacity of the cathode ofexfoliated carbon/graphite materials are significantly higher than thoseof their original carbon/graphite counterparts.

(2) The present invention provides a powerful platform materialsengineering technology for enhancing the specific capacity of carbon orgraphite cathode materials implemented in a multivalent metal-ionbattery.

(3) As demonstrated in FIG. 5, both the Zn-MCMB cell (containing acathode layer of recompressed, exfoliated MCMB) and the Vanadium-CNFcell (containing a cathode of vapor grown carbon nanofibers that havebeen exfoliated and recompressed) show exceptionally stable cycle life,exhibiting less than 20% capacity degradation after 5,000charge-discharge cycles. The cycle life of the presently inventedmultivalent metal-ion cell is typically significantly higher than thecycle life of the lithium-ion battery.

(5) FIG. 6 shows that the specific capacity of a Mg-ion cell containinga cathode layer of favorably oriented, heavily recompressed exfoliatedMCMBs is significantly higher than the specific capacity of a Mg-ioncell containing a cathode of lightly recompressed exfoliated MCMBs. Theedge plane orientation relative to the separator plane plays a criticalrole in dictating the charge storage capability.

(6) By supporting the multivalent metal (in a thin film or coating form)on a nano-structured network composed of interconnected carbon orgraphite filaments (e.g. carbon nanotubes or graphene sheets) one cansignificantly increase the power density and high-rate capability of ametal-ion cell. This is illustrated in FIG. 7, which provides Ragoneplots of four cells: a Mg-ion cell having a CNT-supported Mg anode and acathode layer containing recompressed exfoliated artificial graphite, aMg-ion cell having a Mg foil anode (no CNT support) and a cathode layercontaining recompressed exfoliated artificial graphite, a Ca-ion cellhaving a graphene-supported Ca anode and a cathode layer containingrecompressed exfoliated artificial graphite, and a Ca-ion cell having aCa anode (no graphene support) and a cathode layer containingrecompressed exfoliated artificial graphite. Both Mg-ion cells andCa-ion cells deliver energy densities comparable to those of lithium-ionbatteries, but higher power densities. The power density values arecomparable to those of supercapacitors. In other words, the presentlyinvented multivalent metal-ion batteries have the best of both worlds oflithium-ion batteries and supercapacitors.

This nano-structured network of interconnected carbon nano-fibersprovides large surface areas to support multivalent metal and facilitatefast and uniform dissolution and deposition of metal cations at theanode side. This strategy also has overcome the passivating layer issuecommonly associated with Mg or Ca metal anode. Other nano-filaments ornano-structures that can be used to make such a network includeelectro-spun nano fibers, vapor-grown carbon or graphite nano fibers,carbon or graphite whiskers, carbon nano-tubes, nano-scaled grapheneplatelets, metal nano wires, or a combination thereof.

We claim:
 1. A multivalent metal-ion battery comprising an anode, acathode, a porous separator electronically separating said anode andsaid cathode, and an electrolyte in ionic contact with said anode andsaid cathode to support reversible deposition and dissolution of amultivalent metal, selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr,Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof, at saidanode, wherein said anode contains said multivalent metal or its alloyas an anode active material and said cathode comprises a cathode layerof an exfoliated graphite or carbon material having inter-flake poresfrom 2 nm to 10 μm in pore size.
 2. The multivalent metal-ion battery ofclaim 1, wherein said exfoliated carbon or graphite material in saidcathode active layer is selected from a thermally exfoliated product ofmeso-phase pitch, meso-phase carbon, meso carbon micro-beads (MCMB),coke particles, expanded graphite flakes, artificial graphite particles,natural graphite particles, highly oriented pyrolytic graphite, softcarbon particles, hard carbon particles, multi-walled carbon nanotubes,carbon nano-fibers, carbon fibers, graphite nano-fibers, graphitefibers, carbonized polymer fibers, or a combination thereof.
 3. Themultivalent metal-ion battery of claim 1, wherein said cathode layercomprises an active layer of recompressed exfoliated graphite or carbonmaterial that is oriented in such a manner that said active layer has agraphite edge plane in direct contact with said electrolyte and facingor contacting said separator.
 4. The multivalent metal-ion battery ofclaim 3, wherein said active layer of recompressed exfoliated graphiteor carbon material has a physical density from 0.5 to 1.8 g/cm³ and hasmeso-scaled pores having a pore size from 2 nm to 50 nm.
 5. Themultivalent metal-ion battery of claim 3, wherein said active layer ofrecompressed exfoliated graphite or carbon material has a physicaldensity from 1.1 to 1.8 g/cm³ and has pores having a pore size from 2 nmto 5 nm.
 6. The multivalent metal-ion battery of claim 3, wherein saidactive layer of recompressed exfoliated graphite or carbon material hasa specific surface area from 20 m^(2/)g to 1,500 m^(2/)g.
 7. Themultivalent metal-ion battery of claim 1, further comprising an anodecurrent collector supporting said multivalent metal or its alloy orfurther comprising a cathode current collector supporting said cathodelayer of exfoliated graphite or carbon material.
 8. The multivalentmetal-ion battery of claim 7, wherein said anode current collectorcontains an integrated nano-structure of electrically conductivenanometer-scaled filaments that are interconnected to form a porousnetwork of electron-conducting paths comprising interconnected pores,wherein said filaments have a transverse dimension less than 500 nm. 9.The multivalent metal-ion battery of claim 8, wherein said filamentscomprise an electrically conductive material selected from the groupconsisting of electro-spun nano fibers, vapor-grown carbon or graphitenano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaledgraphene platelets, metal nano wires, and combinations thereof.
 10. Themultivalent metal-ion battery of claim 1, wherein said electrolyte isselected from an aqueous electrolyte, organic electrolyte, polymerelectrolyte, molten salt electrolyte, ionic liquid electrolyte, or acombination thereof.
 11. The multivalent metal-ion battery of claim 1,wherein said electrolyte contains NiSO₄, ZnSO₄, MgSO₄, CaSO₄, BaSO₄,FeSO₄, MnSO₄, CoSO₄, VSO₄, TaSO₄, CrSO₄, CdSO₄, GaSO₄, Zr(SO₄)₂,Nb₂(SO₄)₃, La₂(SO₄)₃, BeCl₂, BaCl₂, MgCl₂, AlCl₃, Be(ClO₄)₂, Ca(ClO₄)₂,Mg(ClO₄)₂, Mg(BF₄)₂, Ca(BF₄)₂, Be(BF₄)₂, Alkyl Grignard reagents,tri(3,5-dimethylphenyl borane, tris(pentafluorophenyl)borane, magnesiumdibutyldiphenyl, Mg(BPh2Bu2)2, magnesium tributylphenyl Mg(BPhBu3)2), ora combination thereof.
 12. The multivalent metal-ion battery of claim 1,wherein the electrolyte comprises at least a metal ion salt selectedfrom a transition metal sulphate, transition metal phosphate, transitionmetal nitrate, transition metal acetate, transition metal carboxylate,transition metal chloride, transition metal bromide, transition metalnitride, transition metal perchlorate, transition metalhexafluorophosphate, transition metal borofluoride, transition metalhexafluoroarsenide, or a combination thereof.
 13. The multivalentmetal-ion battery of claim 1, wherein the electrolyte comprises at leasta metal ion salt selected from a metal sulphate, phosphate, nitrate,acetate, carboxylate, nitride, chloride, bromide, or perchlorate ofzinc, aluminum, titanium, magnesium, calcium, beryllium, manganese,cobalt, nickel, iron, vanadium, tantalum, gallium, chromium, cadmium,niobium, zirconium, lanthanum, or a combination thereof.
 14. Themultivalent metal-ion battery of claim 1, wherein the electrolytecomprises an organic solvent selected from ethylene carbonate (EC),dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate(DEC), methyl butyrate (MB), ethyl propionate, methyl propionate,propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN),ethyl acetate (EA), propyl formate (PF), methyl formate (MF),tetrahydrofuran (THF), toluene, xylene, methyl acetate (MA), or acombination thereof.
 15. The multivalent metal-ion battery of claim 1,wherein the electrolyte also supports reversible intercalation andde-intercalation of ions at the cathode, wherein said ions includecations, anions, or both.
 16. The multivalent metal-ion battery of claim1, wherein said cathode layer of exfoliated carbon or graphite materialoperates as a cathode current collector to collect electrons during adischarge of said multivalent metal-ion battery and wherein said batterycontains no separate or additional cathode current collector.
 17. Themultivalent metal-ion battery of claim 1, wherein said cathode layer ofexfoliated carbon or graphite further comprises a binder material whichbonds said exfoliated carbon or graphite material together to form acathode electrode layer.
 18. The multivalent metal-ion battery of claim17, wherein said binder material is an electrically conductive bindermaterial selected from coal tar pitch, petroleum pitch, meso-phasepitch, a conducting polymer, a polymeric carbon, or a derivativethereof.
 19. The multivalent metal-ion battery of claim 1, wherein saidbattery has an average discharge voltage no less than 1.0 volt and acathode specific capacity greater than 200 mAh/g based on a totalcathode active layer weight.
 20. The multivalent metal-ion battery ofclaim 1, wherein said battery has an average discharge voltage no lessthan 1.0 volt and a cathode specific capacity greater than 300 mAh/gbased on a total cathode active layer weight.
 21. The multivalentmetal-ion battery of claim 1, wherein said battery has an averagedischarge voltage no less than 1.5 volts and a cathode specific capacitygreater than 200 mAh/g based on a total cathode active layer weight. 22.The multivalent metal-ion battery of claim 1, wherein said battery hasan average discharge voltage no less than 1.5 volts and a cathodespecific capacity greater than 300 mAh/g based on a total cathode activelayer weight.
 23. A method of manufacturing a multivalent metal-ionbattery, comprising: (a) providing an anode containing a multivalentmetal or its alloy, wherein said multivalent metal is selected from Ni,Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Mn, V, Co, Fe, Cd, Cr, Ga, In, or acombination thereof; (b) providing a cathode containing a layer ofrecompressed exfoliated carbon or graphite material; and (c) providing aporous separator electronically separating said anode and said cathodeand an electrolyte capable of supporting reversible deposition anddissolution of said multivalent metal at the anode and reversibleadsorption/desorption and/or intercalation/de-intercalation of ions atthe cathode; wherein said layer of recompressed exfoliated carbon orgraphite material is oriented in such a manner that said layer has agraphite edge plane in direct contact with said electrolyte and facingor contacting a plane of said separator.
 24. The method of claim 23,further including providing a porous network of electrically conductivenano-filaments to support said multivalent metal or its alloy.
 25. Themethod of claim 23, wherein providing a cathode contains subjecting acarbon or graphite material to an expansion treatment selected from anoxidation, fluorination, bromination, chlorination, nitrogenation,intercalation, combined oxidation-intercalation, combinedfluorination-intercalation, combined bromination-intercalation, combinedchlorination-intercalation, or combined nitrogenation-intercalation,followed by thermal exfoliation at a temperature from 100° C. to 2,500°C.
 26. The method of claim 23, wherein said procedure of providing thecathode includes compressing exfoliated graphite or carbon using a wetcompression or dry compression to align constituent graphite flakes ofsaid exfoliated graphite or carbon.
 27. The method of claim 23, whereinsaid procedure of providing the cathode includes compressing exfoliatedgraphite or carbon using a wet compression to align constituent graphiteflakes of said exfoliated graphite or carbon, wherein said wetcompression includes compressing or pressing a suspension of exfoliatedgraphite or carbon dispersed in a liquid electrolyte for an aluminumcell.